Technical Field
The present invention concerns a cooling apparatus for subsea applications including a shell and tube heat exchanger.
Offshore platform/topside based processes use coolers extensively. Typically these are based on using seawater as a coolant, often using a “shell and tube” cooler. Such a cooler comprises basically an outer shell, a large diameter pipe that acts as a conduit for the seawater, and closely spaced small diameter tubes that carry the produced fluid through the coolant. The seawater for the cooling is normally drawn from the platform seawater system and hence, no dedicated pumps are necessary. The technology related to shell and tube heat exchangers and the associated pumping arrangement is well established, and has few uncertainties.
Subsea oil and gas development may require cooling to reduce the operational temperature of the produced fluid. Typically, oil and gas pipelines have a design temperature that not should be exceeded. The tie-in of a high temperature development to such a pipeline will require cooling. For a new pipeline/flowline, introduction of cooling to keep the design temperature low, may give operational and cost benefits.
Presently subsea cooling is thus used for flowline/pipeline purposes and coolers for this purpose are based on natural convection to seawater. They are termed passive coolers and comprise a series of tubes, (typically from 1″-6″ OD or 2.54 cm-15.24 cm) exposed to seawater that is allowed to circulate freely.
With the development of subsea gas compression, additional typical subsea cooling requirements have been defined. These requirements include compressor inlet cooling to increase efficiency, where produced fluid is cooled before entering the compressor. Typically subsea produced fluid will leave a wellhead with a temperature of 60° C. to 120° C. Some cooling will take place in the flowlines between the wells and the compression station, but dedicated inlet coolers may be required. Furthermore outlet cooling may be required as the compression process heats the produced fluid and the fluid may have to be cooled prior to entering the flowline/pipeline. Furthermore inter stage cooling may be used in connection with a multi stage compressor to increase the compressor efficiency. The cooling is then introduced between compressor stages. Finally, a compressor anti-surge loop will normally require cooling. Other subsea cooling applications are also conceivable.
Passive coolers as mentioned have been developed for such cooling. While passive convection coolers are simple, they have their limitations. They are large, heavy and their cooling efficiency is difficult to determine by analytical tools. The efficiency of a passive cooler is also sensitive to variations in external and non-controllable parameters as e.g. seabed current and settlement of silt. Furthermore, should the surface temperature of the cooling tubes be high, scaling may form on the tubes. A build-up of scale will reduce the efficiency of the cooler and such scale may be difficult and expensive to remove. A passive cooler also lacks the possibility to control the cooling and its outlet temperature. Typically it might be desirable to keep the outlet temperature above 25° C. to prevent formation of hydrate plugs.
In a topside process plant, seawater pumping and distribution will normally be centralized by use of a piping and valve arrangement. For a subsea application, this may according to the present invention be simplified and based on an inlet strainer per heat exchanger (coarse filtration only), and one pump module per heat exchanger.
It is conceivable to modify a typical passive cooler by arranging a duct around the tubes and add a propeller to increase the circulation. Such an arrangement will alleviate some of the disadvantages of the passive cooler, but do represent an unproven design without the full benefits of using a conventional shell and tube design subsea as outlined below.
Background Art
Such a solution has been suggested in patent application WO 2008/147219 A2. This application relates to a subsea cooling unit for a hot fluid stream from one or more subsea wells, with coils exposed to seawater and a propeller for generating a flow of seawater past the coils. The propeller and coils are arranged in a duct.
Forced convection heat exchangers subsea are known from several publications. This present invention addresses aspects of a support system, and provides a pump module to create forced convection. While retrievable pump systems are well known in the industry, this present invention relates to a system with superior features as outlined below.
Common passive subsea coolers according to the known art offer little or no control over the cooling process. Other non-cooling related, subsea pump systems are controlled by separate power and control modules arranged either subsea or topside.